German patent application DE 195 37 814 A1 discusses the structure of a sensor-layer system and a method for hermetically packaging sensors in surface micromechanics. The manufacture of the sensor structure is based on available technological methods. Capping is accomplished using a separate silicon cap wafer which is structured using elaborate structuring processes such as KOH etching, for example. The sensor cap is applied to the sensor wafer on the wafer level using a glass solder. For this purpose, a wide bond frame is provided on the edge of each sensor chip to ensure adequate bonding and tightness of the cap.
According to the related art, the sensor element is mounted on a lead frame, contacted using gold wires and sheathed using a known injection molding method. In sheathing, the molding compound, liquid plastic for example, flows from the sprue into a hollow mold containing the element to be sheathed.
The thickness and shape of the sensor prove to be disadvantageous in sheathing. This results in an uneven flow and consequently an irregular filling of the cavity above and below the sensor element and to turbulence of the molding compound behind the sensor. These effects may cause undesirable gas bubbles to be enclosed in the molding compound. V. Motta et al. discussed this effect in Micromachining and Microfabrication Process Technology VI, Proceedings of SPIE vol. 4174 (2000) pp. 377-387.
An object was to find a possibility for avoiding air inclusions and turbulence of the molding compound behind the sensor.